The prior art is replete with descriptions of palladium-bearing alloy membranes used to prepare pure hydrogen from gaseous hydrogen-containing mixtures by permeation through a palladium-bearing metallic membrane under pressure and at elevated temperatures. Reference is made to the co-pending patent application Ser. No. 08/719,385 of common assignee which is incorporated herein by reference. Wherein an important advantage of a 60% palladium-40% copper alloy membrane with respect to temperature-cycling and minimal swelling in hydrogen is disclosed. This bears also on the publication by J. Shu, B. P. Grandjean, A. van Neste and S. Kaliaguine, entitled "Catalytic Palladium-based Membrane Reactors: A Review", The Canadian Journal of Chemical Engineering, Vol. 69, (October, 1991), herein also incorporated by reference, wherein this particular palladium-copper alloy is described as showing a sharp maximum at 40 wt % Cu.
Accordingly, for the purpose of the present invention, we use such palladium copper alloys with copper contents sufficiently near the 40% by weight optimum, (i.e. 36-42% Cu), in which narrow range more than two thirds of the maximum flux is retained. Such alloys are termed "Pd/40% Cu" in this specification and the appended claims.
As shown, for example, in the above referenced co-pending application, Pd alloy membranes can be in the form of thin flat foils or small diameter tubes (e.g. 2 mm O.D.). Foils of the Pd/40% Cu membrane are readily available in thicknesses of 0.001 to 0.0025 inches, whereas special expensive techniques a re required to make such Pd/40% Cu tubes with wall thicknesses of no less than 0.0025 inch.
As also disclosed in the above-referenced co-pending application, a palladium alloy foil can be sandwiched between gaskets, and the edge area of the sandwich can be pressed onto a metallic frame. To so produce a leak-tight two-chamber apparatus, a uniform weighty and costly edge pressure is required to at least balance the pressure in the high pressure chamber. It is thus important to replace, in a pure hydrogen generator, the gasket seal with a pressure-tight seal, such as a weld. Welding is preferred over brazing or soldering to avoid contamination of the foil by the extraneous metals of the latter.
Diffusion welding employs temperatures that range from 50 to 75% of the melting point "Procedure Development and Practice Considerations for Diffusion Welding", by S. B. Dunkerton, "ASM Handbook", vol. 6, p. 883, ASM International (1993)!. Diffusion welding of copper to a different metal or to an alloy "is conducted at a temperature greater than one-half of the absolute melting point" Diffusion welding of Solid-State Welding, by J. L. Jellison and F. J. Zanner, "Metals Handbook", 9th Ed. Vol. 6, p. 672, ASM International (1983); see first column on page 672 and Table 1 on page 677!.
The Pd/40% Cu alloy has a melting point of approximately 1200.degree. C. (1473K) "ASM Handbook", vol. 3, p. 717, ASM International (1993)!. Hence welding it to copper is expected to require a temperature in excess of about 460.degree. C. (733K). However, we have found that the hydrogen flux across the Pd/40% Cu foil deteriorates after the foil has been exposed to such high temperatures.
Underlying the present invention is the discovery that these problems can be overcome by providing a hydrogen selective Pd/40% Cu membrane wall in the form of a thin palladium-copper alloy membrane of carefully controlled composition in an open-area copper-surfaced metallic frame, hermetically bonded by diffusion-bonding the membrane to the frame.
The term "diffusion-bonding", as used herein means controlling and maintaining an elevated bonding temperature below about 350.degree. C., while subjecting the edge area of the membrane in contact with the frame to a substantially uniform high pressure, in an oxygen-free atmosphere.